1. Field of the Invention
This invention is in the field of seismic surveying, and is more specifically directed to the evaluation of the quality of acquisition of three dimensional seismic data in an onshore or offshore environment.
2. Background of the Art
In reflection seismology, acoustic waves are imparted into the earth, generally by activation of a seismic source. Acoustic receivers detect the acoustic waves after their reflection from sub-surface strata and interfaces. Analysis of the acoustic waves, together with the known position of the source and receiver is used to provide an image of the subsurface.
In conventional 2-D surveys, the acoustic receivers are deployed in a line and the source is collinear with the receivers. In marine seismic surveys, 2-D surveys are generally performed by a vessel that tows a source, such as an air gun, followed by a streamer of hydrophones. Other types of 2-D marine surveys use receivers deployed on the ocean bottom with the source being towed by a vessel. A 2-D survey provides survey information relative only to a vertical plane into the earth.
Most seismic surveys carried out at present are three-dimensional (3-D) surveys. The surveying of an area by way of multiple parallel 2-D surveys has been referred to in the art as xe2x80x9c3-Dxe2x80x9d seismic surveys, as an image of a subsurface region of the earth is generated. However, this type of survey is not truly xe2x80x9c3-D,xe2x80x9d as information is acquired only at two source-receiver angles, or azimuths, with these two azimuths at a 180xc2x0 angle relative to each other. If the lines of a multiple 2-D survey are sufficiently close together, it is possible to process the data to account for the fact that the reflections of the seismic waves do not originate from a vertical plane through the lines of the survey.
Conventional seismic analysis techniques xe2x80x9cstackxe2x80x9d multiple traces of acoustic amplitude versus time for midpoints within a certain vicinity of the same location (such vicinity generally referred to as the xe2x80x9cbinxe2x80x9d), reinforcing the xe2x80x9csignalxe2x80x9d portion of the traces while the random acoustic xe2x80x9cnoisexe2x80x9d tends to cancel out. As is well known, it is preferable that the multiple stacked traces for a given bin correspond to varying source-receiver offset distances, with normal move-out (xe2x80x9cNMOxe2x80x9d) and/or dip move-out (xe2x80x9cDMOxe2x80x9d) operations adjusting for the difference in travel time versus offset for a particular midpoint. Such stacking, or gathering, of trace data is conventionally referred to as common depth point (xe2x80x9cCDPxe2x80x9d) or common midpoint (xe2x80x9cCMPxe2x80x9d) stacking, with the number of traces for a given bin generally referred to as the xe2x80x9cfoldxe2x80x9d of the survey for that bin.
It is preferable for proper imaging of the subsurface that the seismic wavefield be uniformly sampled areally. Some processing steps such as stacking, multi-channel filtering, DMO correction, prestack migration, velocity analyses, anisotropy studies and wavefield extrapolation require that the data be uniformly sampled in offset and/or azimuths. This ideal sampling distribution may be difficult to obtain in land and marine seismic surveys at a reasonable cost. Even in the absence of obstructions such as platforms, a strong cross current will cause a degradation in the sampling of different offsets and azimuths in a marine seismic survey. In addition, in land seismic surveys, obstructions such as buildings, roads, or gaps in coverage where the landowner refuses to give a permit for surveying further complicate the acquisition. In marine seismic surveys, obstructions such as platforms make the acquisition of data with uniform sampling difficult or impossible. In conventional marine surveys with detectors located on streamers towed by a ship, the obstruction makes it impossible for the ship to follow a path that gives uniform coverage. In ocean bottom surveys where the data are recorded on sensors located on the ocean bottom, it is difficult to lay down detectors near obstructions.
If the sampling is too sparse, then processing methods such as migration and k-f filtering will produce artifacts due to aliasing of the data. If the sampling does not adequately cover a sufficiently large range of offsets, then the processing methods will not adequately suppress noises in the data. It is therefore desirable to have an invention in which an acquisition geometry for 3-D seismic data acquisition can be evaluated to determine its adequacy in terms of suppression of noise and absence of aliasing. The present invention satisfies the need.
A planned survey comprises several lines of receivers that detect seismic reflections produced by sources on a plurality of lines. In a conventional marine survey, the sources are deployed from ships and the detectors are on streamer cables towed by the ship. In ocean bottom surveys, the detectors are deployed at the bottom of a body of water and a source vessel travels along a planned series of paths periodically firing a conventional seismic source; the detectors may be on a cable for carrying signals received by the detectors or may be provided with transmitters for sending the signals by telemetry to a remote location for further processing. The survey area is divided into a number of xe2x80x9cbinsxe2x80x9d of a convenient shape and size, such as squares with a side equal to half the receiver spacing (when the receivers are equally spaced on the receiver lines). The data from the suite of shot and receiver positions are analyzed and all shot-receiver pairs that have a common sort point are assigned to the spatial bin in which the sort point is located. For P- wave prospecting, the sort point is midway between the source and the receiver and the sort point is a CMP. For converted wave surveying, the sort point is usually between one-half and two-thirds of the distance from the source to the receiver. The data within each bin are analyzed to give a figure of merit of the adequacy of the sampling for each bin. These figures of merit are displayed in a color display that makes the inadequacies of the sampling apparent, making it possible to modify the planned acquisition geometry prior to the actual acquisition. Those versed in the art would recognize that it is not necessary for each of the receiver locations in a survey to be recording data corresponding to seismic waves transmitted from each of the source locations in the survey.